The present invention relates to the field of thermal spray technologies for applying coatings and in particular to detonation thermal spray.
The object of the present invention is a gas feeder apparatus for a detonation spray gun which provides a high safety of use as well as a greater productivity and versatility.
At this time, detonation spray technology is mainly used to apply coatings to workpieces exposed to severe wear, heat or corrosion and is fundamentally based on using the kinetic energy produced in the detonation of combustible mixtures of gases to deposit powdered coating materials on workpieces.
Coating materials typically used in detonation processes include powder forms of metals, metal-ceramics and ceramics and are applied to improve resistance to wear, erosion, corrosion, as thermal insulators and as electrical insulators or conductors.
Spraying by detonation is performed by spray guns which basically consist of a tubular detonation chamber, with one closed end and one open end, to the latter being attached an also tubular barrel. A combustion mixture is injected into the detonation chamber and ignition of the gas mixture is achieved with a spark plug, causing a detonation and consequently a shock or pressure wave which travels at supersonic speeds inside the chamber and then inside the barrel until it leaves through the open end of the barrel.
The coating material powder is generally injected into the barrel in front of the propagating shock wave front and is then carried out to the open end of the barrel and deposited onto a substrate or workpiece placed in front of the barrel. The impact of the coating powder onto the substrate produces a high-density coating with good adhesive characteristics.
This process is repeated cyclically until the workpiece is adequately coated.
In a typical detonation gun, the gases which make up the mixture to be detonated, oxygen and a fuel such as natural gas, propane, propylene, hydrogen or acetylene are mixed before they enter the detonation chamber in a mixing chamber, to ensure the homogeneity of the mixture in the detonation chamber at the time of explosion. The chamber or conduits in which the gases are mixed make up a volume in which flame and shock wave returns must be absent, to prevent backfiring into the fuel and oxygen supplies. This basic safety requirement is solved in traditional devices in three basic ways:
a) Detonation systems in which the mixing chamber, the detonation chamber and the gas feeding supplies are isolated by a valve system synchronized with the firing system. In this arrangement, valves open to allow the gases to pass into the premixture chamber and from it to the detonation chamber and close during the explosion to isolate the feeding supplies from the detonation chamber. Devices of this type are described in U.S. Pat. Nos. 4,687,135 and 4,096,945.
This is a solution widely used but its main disadvantage refers to the fact that the valve system complicates the apparatus and uses mechanical moving parts, which causes reliability problems and limits the productivity. In these devices, the detonation wave is stopped from advancing by filling the mixing chamber with an inert gas such as nitrogen or a noble gas which prevents propagation inside it.
b) U.S. Pat. No. 4,258,091 refers to a method for applying coatings in which the fuel gases are fed continuously into a mixing chamber and from there they pass, through a pipe, into the detonation chamber. To achieve a cyclically and controlled feeding of the mixed gases into the detonation chamber, an inert gas is fed to an intermediate area of the communication pipe between the mixing chamber and the detonation chamber. The injection of the inert gas into the pipe is controlled cyclically by a valve, so that volumes of gas mixture and inert gas arrive in an alternate manner at the detonation chamber. The volume of inert gas allows controlling the adequate mixture volume for detonation and also prevents backfiring into the mixing chamber.
The main disadvantage of this device is its low productivity.
c) Detonation apparatus in which the mixing chamber is communicated with the detonation chamber by a labyrinth-like tortuous path or conduit, which precludes the propagation of the detonation wave by collision of the detonation cells, which make up the shock wave, against the labyrinth walls, so that the wave loses enough pressure not to be able to propagate through the gas feeding supplies. Such an apparatus is described in PCT Patent US96/20160 of the applicant.
In this case, the tortuous path or labyrinth presents a particular geometry which depends on the composition of the gas mixture, since the size of the detonation cells depends on the mixture, and so the labyrinth must be specifically designed to cause the annihilation of the cells which propagate in it. This has the disadvantage that the equipment is designed to annihilate cells corresponding to certain fuel mixtures; a new labyrinth design or, at best, a rearrangement of its geometry is required for safe use with a different gas mixture, which generates cells of different size.
Even for a same pair of gases the labyrinth design can only ensure safety of the system in a limited composition interval of the mixture and pressure of the gases in the combustion chamber.
Another important disadvantage of this type of systems relates to the fact that since there is free communication between the detonation chamber and the mixing chamber it is not possible to completely eliminate backfiring into the mixing chamber, so that between successive detonations there is a combustion of gases contained in the latter. When these gases burn inside the mixing chamber, ashes and soot are created which are deposited on the chamber walls and on the gas feeding conduits, possibly even obstructing these, so that it is necessary to periodically clean and maintain these.
A similar solution to the above one and therefore with the same disadvantages mentioned is described in U.S. Pat. 5,542,606. In this Patent, combustion of the gases occurs in the gas mixing chamber itself, propagating through narrow conduits until a larger chamber is reached where the detonation occurs.
The present invention fully solves the above disadvantages by a continuous gas feeding system which communicates directly and separately the oxygen and fuel gases supplies with the detonation chamber without there being an intermediate chamber or conduit where the fuel gases and oxygen mix before they arrive at the detonation chamber.
The apparatus of the invention have no valves or moving parts to close communication between the detonation chamber and the gas feeding supplies and consists only of a series of independent passages for each of the gases, the design and size of which allow obtaining cyclical detonations with a continuous gas feeding, in addition guaranteeing a fast and thorough distribution of gases in the detonation chamber to obtain a fast and efficient homogeneity of the mixture.
More specifically, each of the independent passages which communicate the feeding supplies to the detonation chamber consists of an expansion chamber and a number of distribution conduits of small cross section and/or great length, so that each gas arrives at the detonation chamber separated from the other gas and through a number of small orifices, as in a shower head, guaranteeing a correct spatial distribution of the gases inside the detonation chamber and thereby a proper homogeneity of the mixture produced in the detonation chamber prior to the explosion.
Once the detonation occurs, the pressure wave generated travels in all directions, mainly through the barrel, but also through the multiple gas distribution passages which open into the detonation chamber. Due to the geometry of these, the progression of the gases through the distribution passages takes place with difficulty so that the gases lose a great deal of heat by thermal transmission to the outer surface of the conduits, cooling down to a temperature below that of ignition of the mixture.
After this, when the main volume of detonation gases passes out through the barrel, the gases which traveled in the distribution conduits are suctioned in, returning already cooled to the detonation chamber, forming a volume of cold gases which is located immediately behind the hot detonation gases, thus acting as a thermal barrier between the very hot detonated gases and the new volume of gases which enters the chamber for a new detonation cycle. This volume of cold gases prevents the detonated gases from being in direct contact with the new volume of gases, thus avoiding the propagation of combustion to the new gases, that is, the cooled detonated gases inside the distribution conduits act as a barrier separating cyclically volumes of gases which cause combustion and therefore detonate cyclically.
As has been exposed, this injection system based on a set of independent passages, consisting of a number of conduits of reduced cross section and/or great length, converts a continuous feeding of gases into cyclical detonations inside the detonation chamber.
In addition, the device also acts as a safety valve, preventing the pressure wave from reaching the gas feeding supplies after each explosion since the special geometry of the distribution conduits makes the gas advance slowly inside them, so that before the pressure wave front reaches the feeding supplies all the explosion volume has left through the barrel and therefore the pressure of the wave rapidly disappears.
Nevertheless, the system is intrinsically safe as there is no volume of explosive mixture, oxygen and combustion gas, in any chamber or conduit of the device except the detonation chamber. This means that even in the case of backfiring, there would be no serious consequences as neither the oxygen nor the fuel (except acetylene) can burn on their own, much less explode.
With the system described, the spray frequency is greater than in present equipment due to the fact that there are no moving parts and it is not necessary to refill the gas and oxygen volumes of the mixing chamber between successive discharges which in other systems are lost through combustion. This means that a faster refill of the detonation chamber can be obtained and therefore a higher working frequency can also be obtained.
The apparatus of the invention is placed directly between the gas feeding supplies and the detonation chamber and can be made in the walls of the chamber itself, as a rod or cylinder placed axially behind the chamber, or preferably as one or several caps internally connected to the detonation chamber. When the expansion chambers are placed around the perimeter of the aforementioned caps, they may occupy an arc of circumference or the full circumference, where in the first case the feeding lines must be arranged radially with respect to the detonation chamber.
Finally, the described system shows greater flexibility than known systems in that there is no limitation as far as the type of gas to be used, in other words, it is not necessary to adapt or modify the detonation gun even if different gases or mixtures of gases are used.